Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: July 12, 2011 Kirei AB
January 8, 2011
Using Secure DNS to Associate Certificates with Domain Names For TLS
draft-ietf-dane-protocol-01
Abstract
TLS and DTLS use certificates for authenticating the server. Users
want their applications to verify that the certificate provided by
the TLS server is in fact associated with the domain name they
expect. Instead of trusting a certification authority to have made
this association correctly, the user might instead trust the
authoritative DNS server for the domain name to make that
association. This document describes how to use secure DNS to
associate the TLS server's certificate with the the intended domain
name.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 12, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Hoffman & Schlyter Expires July 12, 2011 [Page 1]
Internet-Draft DNS Cert Linkage for TLS January 2011
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
The first response from the server in TLS may contain a certificate.
In order for the TLS client to authenticate that it is talking to the
expected TLS server, the client must validate that this certificate
is associated with the domain name used by the client to get to the
server. Currently, the client must extract the domain name from the
certificate, must trust a trust anchor upon which the server's
certificate is rooted, and must successfully validate the
certificate.
Some people want a different way to authenticate the association of
the server's certificate with the intended domain name without
trusting the CA. Given that the DNS administrator for a domain name
is authorized to give identifying information about the zone, it
makes sense to allow that administrator to also make an authoritative
binding between the domain name and a certificate that might be used
by a host at that domain name. The easiest way to do this is to use
the DNS.
This document applies to both TLS [RFC5246] and DTLS [4347bis]. In
order to make the document more readable, it mostly only talks about
"TLS", but in all cases, it means "TLS or DTLS". This document only
relates to securely associating certificates for TLS and DTLS with
host names; other security protocols are handled in other documents.
For example, keys for IPsec are covered in [RFC4025] and keys for SSH
are covered in [RFC4255].
1.1. Certificate Associations
In this document, a certificate association is based on a
cryptographic hash of a certificate (sometimes called a
"fingerprint") or on the certificate itself. For a fingerprint, a
hash is taken of the binary, DER-encoded certificate, and that hash
is the certificate association; the type of hash function used can be
chosen by the DNS administrator. When using the certificate itself
in the certificate association, the entire certificate in the normal
format is used. This document also only applies to PKIX [RFC5280]
certificates.
Certificate associations are made between a certificate or the hash
Hoffman & Schlyter Expires July 12, 2011 [Page 2]
Internet-Draft DNS Cert Linkage for TLS January 2011
of a certificate and a domain name. Server software that is running
TLS that is found at that domain name would use a certificate that
has a certificate association given in the DNS, as described in this
document. A DNS query can return multiple certificate associations,
such as in the case of different server software on a single host
using different certificates (even if they are normally accessed with
different host names), or in the case that a server is changing from
one certificate to another.
1.2. Securing Certificate Associations
This document defines a secure method to associate the certificate
that is obtained from the TLS server with a domain name using DNS
protected by DNSSEC. Because the certificate association was
retrieved based on a DNS query, the domain name in the query is by
definition associated with the certificate.
DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
[RFC4034], and [RFC4035]), uses cryptographic keys and digital
signatures to provide authentication of DNS data. Information
retrieved from the DNS and that is validated using DNSSEC is thereby
proved to be the authoritative data. The DNSSEC signature MUST be
validated on all responses in order to assure the proof of origin of
the data.
This document only relates to securely getting the DNS information
for the certificate association using DNSSEC; other secure DNS
mechanisms are out of scope.
1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
A note on terminology: Some people have said that this protocol is a
form of "certificate exclusion". This is true, but in a very unusual
sense. That is, a DNS reply that contains two of the certificate
types defined here inherently excludes every other possible
certificate in the universe other than those found with a pre-image
attack against one of those two. The certificate type defined here
is better thought of as "enumeration" of a small number of
certificate associations, not "exclusion" of a near-infinite number
of other certificates.
Some of the terminology in this draft may not match with the
terminology used in RFC 5280. This will be fixed in future versions
of this draft, with help from the PKIX community. In specific, we
Hoffman & Schlyter Expires July 12, 2011 [Page 3]
Internet-Draft DNS Cert Linkage for TLS January 2011
need to say (in a PKIX-appropriate way) that when we say "valid up
to" and "chains to", full RFC 5280 path processing including
revocation status checking is intended.
2. Getting TLS Certificate Associations from the DNS
This document defines a new DNS resource record type, "TLSA". A
query on a domain name for the TLSA RR can return one or more records
of the type TLSA. The TLSA RRType is TBD.
The format of the data in the resource record is a binary record with
three values, which MUST be in the order defined here:
o A one-octet value, called "certificate type", specifying the
provided association that will be used to match the target
certificate. The types defined are:
1 -- Hash of an end-entity certificate
2 -- Full end-entity certificate in DER encoding
3 -- Hash of an certification authority's certificate
4 -- Full certification authority's certificate in DER encoding
o A one-octet value, called "hash type", specifying the type of hash
algorithm used for the certificate association. This value has
the same values as those of the TLS hash, as defined in the IANA
registry titled "TLS HashAlgorithm Registry"
(<http://www.iana.org/assignments/tls-parameters>). For example,
the value for the SHA-1 hash function is "2". When no hashing is
used (that is, in the certificate types where the full certificate
is given), the hash type is 0. Using the same hash algorithm as
is used in the signature in the certificate will make it more
likely that the TLS client will understand this TLSA data. [[
Note: this is currently being discussed in the WG as issue #4, so
it could change. ]]
o The bytes containing the certificate or the hash of the associated
certificate (that is, the certificate or the hash of the
certificate itself, not of the TLS ASN.1Cert object).
Certificate types 1 through 4 explicitly only apply to PKIX-formatted
certificates. If TLS allows other formats later, or if extensions to
this protocol are made that accept other formats for certificates,
those certificates will need certificate types. [[ Later: maybe make
yet-another-probably-never-used IANA registry for certificate types.
Hoffman & Schlyter Expires July 12, 2011 [Page 4]
Internet-Draft DNS Cert Linkage for TLS January 2011
]]
2.1. Making Certificate Associations
The TLS client determines whether or not the certificate offered by
the TLS server matches the certificate association in the TLSA
resource record. If the certificate from the TLS server matches, the
TLS client accepts the certificate association. Each certificate
type has a different method for determining matching.
For types 1 and 3, the hash used in the comparison is the hash type
from the TLSA data.
Types 1 (hash of an end-entity certificate) and 2 (full end-entity
certificate) are matched against the first certificate offered by the
TLS server. For type 1, the certificate association is valid if the
hash of the first certificate offered by the TLS server matches the
value from the resource record. For type 2, the certificate
association is valid if the certificate in the TLSA data matches to
the first certificate offered by TLS.
Type 3 (hash of certification authority's certificate) can be used in
one of two ways. If the hash of any certificate past the first in
the certificate bundle from TLS matches the value from the TLSA data,
and the chain in the certificate bundle is valid up to that
certificate, then the certificate association is valid. Alternately,
if the first certificate offered chains to a trust anchor, and the
hash of that trust anchor matches the value from the TLSA data
(assuming that the trust anchor is kept in certificate format), then
the certificate association is valid.
Type 4 (full certification authority's certificate) is used in
chaining from the end-entity given in TLS. The certificate
association is valid if the first certificate in the certificate
bundle can be validly chained to the certificate from the TLSA data
(assuming that the trust anchor is kept in certificate format).
[[ Need discussion of self-signed certificates being CA certificates.
Need to be sure that this discussion uses correct PKIX terminology
and is carefully explained. ]]
2.2. Presentation Format
The RDATA of the presentation format of the TLSA resource record
consists of two numbers (certificate and hash type) followed by the
bytes containing the certificate or the hash of the associated
certificate itself, presented in hex. An example of a hash of an
end-entity certificate:
Hoffman & Schlyter Expires July 12, 2011 [Page 5]
Internet-Draft DNS Cert Linkage for TLS January 2011
www.example.com. IN TLSA (
1 2 e77b719d4e9c63c0b0a0333be0a4188e490b618e )
The use of mnemonics instead of numbers is not allowed.
[[ We could consider using Base64 instead of hex. ]]
2.3. Wire Format
[[ Need to do this, clearly. ]]
3. Use of TLS Certificate Associations in TLS
In order to use one or more TLS certificate associations described in
this document obtained from the DNS, an application MUST assure that
the certificates were obtained using DNS protected by DNSSEC. TLSA
records must only be trusted if they were obtained from a trusted
source. This could be a localhost DNS resolver answer with the AD
bit set, an inline validating resolver library primed with the proper
trust anchors, or obtained from a remote nameserver to which one has
a secured channel of communication.
If a certificate association contains a hash type that is not
understood by the TLS client, that certificate association MUST be
marked as unusable.
An application that requests TLS certificate associations using the
method described in this document obtains zero or more usable
certificate associations. If the application receives zero usable
certificate associations, it processes TLS in the normal fashion.
If a match between one of the certificate association(s) and the
server's end entity certificate in TLS is found, the TLS client
continues the TLS handshake. If a match between the certificate
association(s) and the server's end entity certificate in TLS is not
found, the TLS client MUST abort the handshake with an
"access_denied" error.
3.1. Certificate Validation by TLS Clients When Using Certificate
Associations
TLS client policy is deliberately not prescribed by this
specification. A client MAY choose to trust a DNSSEC-secured
certificate association, depending on its local policy.
[[ The preceding paragraph is probably wrong in the sense that it
means that we now have no conformance requirements. There is
Hoffman & Schlyter Expires July 12, 2011 [Page 6]
Internet-Draft DNS Cert Linkage for TLS January 2011
probably no reason to even use this protocol unless you are going to
fully trust the results. The one exception that has been discussed
is that you might want to use the TLSA data as a "second positive
opinion", such as in a GUI or in logging. Both of those seem fairly
useless in the case of DNS resolution. Thus, the above paragraph may
be changed by the WG in a future version of this draft. ]]
3.1.1. Use of Self-Signed Certificates
One expected use case for this protocol is that some TLS servers will
begin to use self-signed certificates in association with certificate
associations. A TLS client that is using this protocol needs to
treat self-signed certificates as special, and thus SHOULD NOT
attempt certificate validation on them. (An exception to this rule
would be clients that keep self-signed end entity certificates in its
trust anchor store.)
3.1.2. Ignorning Host Names in Self-Signed Certificates
All data in a self-signed certificate other than the key itself can
be ignored as untrusted unless a client validates the self-signed
certificate to a trust anchor that is identical to the certificate.
That means that the host name given in the self-signed certificate is
meaningless, and that the only way to associate the public key in the
certificate with the domain name is through the certificate
association made in the DNS.
If a TLS client fully trusts the association between a domain name
and the certificate that was provided by the DNS, then that client
MUST ignore the domain name that is given in the self-signed
certificate. That is, the certificate might contain a domain name
that is different than the one that was used to get the TLSA data,
but if the client is trusting the TLSA data, it doesn't matter what
domain name is used in the certificate. An expected use case for
this protocol is to allow someone who controls the private key on a
certificate to use that certificate for multiple TLS servers. These
servers might be on a single computer that has many domain names
(such as a computer that is both a web host and a mail host, and is
known by both "www.example.com" and "smtp.example.com"), or they
might be on different computers (such as multiple computers that all
respond IP addresses reachable as "www.example.com").
[[ Add more about virtual hosting and SNI TLS extension. ]]
3.1.3. Use of Local Trust Anchors
Another expected use case for this protocol is that some TLS servers
will use certificates that chain to a trust anchor that might not be
Hoffman & Schlyter Expires July 12, 2011 [Page 7]
Internet-Draft DNS Cert Linkage for TLS January 2011
one that is trusted by the TLS client, such as a local certification
authority (CA) that is administered by the organization that runs the
TLS server; this is a likely use for certificate types 3 and 4.
Because of this, a TLS client that is using this protocol that
performs certificate validation on server certificates MAY have a
method to communicate with the user that differentiates between
validation failures that occur on certificates that have had secure
certificate associations and those that have not. If it does not
have such a method of communication, the failure to validate SHOULD
cause the same error as for any other certificate validation.
3.1.4. Use of Additional Certificate Data
Some TLS clients extract data from the certificate other than the key
to show to the user; for example, most modern web browsers have the
ability to show an extended validation (EV) name that is embedded in
a certificate. Because this data comes from a trusted third party
and not the TLS server itself, TLS clients that extract additional
information from TLS server certificates MUST validate those
certificates in the normal fashion.
4. IANA Considerations
This document uses a new DNS RRType, TLSA, whose value is TBD. A
separate request for the RRType will be submitted to the expert
reviewer, and future versions of this document will have that value
instead of TBD.
5. Security Considerations
The security of the protocols described in this document relies on
the security of DNSSEC as used by the client requesting A and TLSA
records.
A DNS administrator who goes rogue and changes both the A and TLSA
records for a domain name can cause the user to go to an unauthorized
server that will appear authorized, unless the client performs
certificate validation and rejects the certificate. That
administrator could probably get a certificate issued anyway, so this
is not an additional threat.
The values in the TLSA data will be normally entered in the DNS
through the same system used to enter A/AAAA records, and other DNS
information for the host name. If the authentication for changes to
the host information is weak, an attacker can easily change any of
this information. Given that the TLSA data is not easily human-
Hoffman & Schlyter Expires July 12, 2011 [Page 8]
Internet-Draft DNS Cert Linkage for TLS January 2011
readable, an attacker might change those records and A/AAAA records
and not have the change be noticed if changes to a zone are only
monitored visually.
If the authentication mechanism for adding or changing TLSA data in a
zone is weaker than the authentication mechanism for changing the
A/AAAA records, a man-in-the-middle who can redirect traffic to their
site may be able to impersonate the attacked host in TLS if they can
use the weaker authentication mechanism. A better design for
authenticating DNS would be to have the same level of authentication
used for all DNS additions and changes for a particular host.
[[ Add discussion of the idea that TLSA makes things worse if an
intermediate CA is compromised. Need more from Stephen Farrell. ]]
[[ Add discussion of length check to avoid potential issues with
appended data. Need more from Carl Wallace. ]]
6. Acknowledgements
Many of the ideas in this document have been discussed over many
years. More recently, the ideas have been discussed by the authors
and others in a more focused fashion. In particular, some of the
ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley,
Ilari Liusvaara, and Ondrej Sury.
7. References
7.1. Normative References
[4347bis] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
progress), July 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
Hoffman & Schlyter Expires July 12, 2011 [Page 9]
Internet-Draft DNS Cert Linkage for TLS January 2011
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
7.2. Informative References
[RFC4025] Richardson, M., "A Method for Storing IPsec Keying
Material in DNS", RFC 4025, March 2005.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
Hoffman & Schlyter Expires July 12, 2011 [Page 10]
